1. Field of the Invention
The present invention relates to a method for forming a coating with various kinds of liquid compositions (hereinafter referred to as "a coating liquid") onto a continuously running belt-like support (hereinafter referred to as "a web") used in manufacturing photosensitized material film; photographic print paper; magnetic recording materials such as magnetic recording tapes, etc.; adhesive tape; information recording paper such as pressure-sensitive paper, heat-sensitive paper, etc.; other photographic printing plate, and so on. More particularly, the present invention relates to a curtain coating method and an apparatus therefor.
2. Description of the Related Art
A curtain coating method and an apparatus therefor are representative of a method and apparatus in which a coating liquid in the form of a thin film is made to collide with a running web so as to coat the running web with the coating liquid.
Curtain coating method and apparatus therefor include a method and apparatus wherein a freely falling curtain film composed of one or more kinds of coating liquids is made to collide with an object to be coated so as to form a coating film on the object. The basic curtain coating method and apparatus therefor have been used for many years for coating furniture, iron plates, etc. Recently, however, the curtain coating method has been applied to a field in which accuracy is particularly required, such as in manufacturing photosensitized materials, as disclosed in U.S. Pat. No. 3,632,374 or U.S. Pat. No. 3,508,947.
In such a method and apparatus, it is extremely important to evenly apply the freely falling curtain film at the time coating is started.
Compared with a slide-hopper-type bead coating method, the curtain coating method is required to perform coating at a higher speed. Therefore, the flow of the fed liquid is required to be increased correspondingly. As a result, forming an even coating becomes difficult at the start of coating. In the case where even coating cannot be performed, problems may occur in products because of contamination due to spattering of the coating liquid, contamination of a roller in the stage after a drying zone due to a not-yet-dried thickly-coated portion, or the like.
A curtain coating method is disclosed, for example, in Japanese Patent Publication No. Sho-49-24133, in which a rotatable or slidable device called a deflector (coating plate) is used for dual purposes, namely, for forming a stable curtain film to feed a predetermined quantity of coating at the start of coating and for collecting the coating liquid prior to the start of coating.
FIG. 4 shows the coating method disclosed in U.S. Pat. No. 3,632,374. A coating liquid 13 made to flow onto a slide surface 12 of a slide hopper 11 and then freely falls in the form of a thin film from a forward end portion 14 of the slide surface 12 to form a curtain film 15. The curtain film 15 is made to collide with a running web 16 so as to form a coating thereon. Before coating is started, a rectangular plane coating plate 18 is inserted so as to interrupt the curtain film 15 so that the coating liquid flows along the coating plate 18 so as to be collected in a collecting tank 21. At the start of coating, the coating plate 18 is rotated about a fulcrum 19 so that the curtain film 15 reaches the web 16 to thereby perform coating. Further, the opposite side ends of the curtain film 15 are held by edge guides 20 extending from the forward end portion 14 of the slide surface 12 to a position lower than the position at which the curtain film 15 collides with the web.
This method has an effect that, in the case of single layer coating, the free falling curtain film 15 is received by the coating plate 18 prior to the start of coating so that the coating liquid can be collected in the collecting tank 21 so as to be used again. However, the apparatus has the following three defects. The first defect is that the curtain film 15 is required to be sufficiently high in order to rotate or slide the coating plate 18 and, therefore, the use of the apparatus is limited in the case where there is not sufficient space. The second defect is that non-coating portions exist at the opposite side ends of the curtain film 15 between the edge guides 20 and the coating plate 18. The third defect is that although the coating liquid is applied onto the entire width of the web 16 at the same time, at the instance when the rotating and retreating coating plate 18 is separated from the curtain film 15 at the start of coating a thickly coated portion is formed with the coating liquid lump portion.
U.S. Pat. No. 4,922,851 and U.S. Pat. No. 4,851,268 disclose a coating apparatus which attempts to overcome the above-mentioned defects. Namely, the coating plate is caused to effectively function to realize smooth coating, that is, coating without the formation of thick portions, even in the case where film formation is difficult in the slide-hopper-type curtain coating apparatus. In this apparatus, an L-shaped coating plate 9 as shown in FIGS. 3(a) and 3(b) is used in a manner so that the sliding speed of the coating plate 9 can be suitably selected.
However, in the method and apparatus disclosed in the above U.S. Pat. No. 4,922,851 and U.S. Pat. No. 4,851,268, when the coating plate 9 is removed, a freely falling coating-liquid film 5 wets the lower surface of a forward end portion of the coating plate 9 as shown in FIG. 3(a) so that the liquid is finally moved onto the web 6 together with the liquid to be applied onto the web 6 so as to form an excessive bead portion 22 as shown in FIG. 3(b), and a part of the bead portion 22 becomes a thickly coated portion at the start of coating, while another part swings so as to make the bead unstable, resulting in unevenness in the coated film thickness. Assume that a point where the freely falling coating-liquid film 5 falls down onto the web 6 is a coating point and that an angle (hereinafter referred to as "coating angle") between a vertical line and a straight line starting from the center of rotation of a backup roller to the coating point is .PSI., coating has been difficult particularly in the case where the coating angle .PSI. is larger than 30 degrees in the conventional method and apparatus.